4IO THE DISTRIBUTION OF ANIMALS AND PLANTS 



part of the continents, and may be portions of " ancient and 

 widespread groups," once widely diffused, but now restricted 

 to a few insular spots. Among the land snails, if anywhere, 

 we should find evidence either of autochthonous evolution or 

 of specific change. These animals have existed on the earth 

 since the Carboniferous period, and, notwithstanding their 

 proverbial slowness and sedentary habits, they have contrived 

 to colonize every habitable spot of land on the globe — that is, 

 unless in some of these places they have originated de novo. 

 In the Azores there are sixty-nine species of land snails, of 

 which no less than thirty-two, or nearly one-half, are peculiar, 

 though nearly all are closely allied to European types. What, 

 then, is the origin of these thirty-two species, admitting for the 

 sake of argument that they are really distinct, and not merely 

 varietal forms, though it is well known that in this group species 

 are often unduly multiplied. Three suppositions are possible, 

 (i) These snails may have originated in the islands themselves, 

 either by creation or evolution from lower forms ; say, from sea 

 snails. (2) They may have been modified from modern con- 

 tinental species. (3) They may be unmodified descendants 

 of species of Miocene or Pliocene age now existing on the 

 continents only as fossils. As the islands appear to have ex- 

 isted since Miocene times, it is no more improbable that 

 species of that or the Pliocene age should have found their 

 way to them than that modern species should ; and as we 

 know only a fraction of the Tertiary species of Europe or 

 Africa, it is not likely that we shall be able to identify all of 

 these early visitors. Unfortunately no Miocene or Pliocene 

 deposits holding remains of land snails are known in the 

 Azores themselves, so that this kind of evidence fails us. In 

 Madeira and Porto Santo, however, where there are numerous 

 modern snails, there are Pliocene beds holding remains of these 

 animals. In Madeira there are, according to Lyell, 36 Plio- 

 cene species, and in Porto Santo 35, and of these only eight 



